ISSN: 2615-9740
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Ho Chi Minh City University of Technology and Education
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Analyzing the Sources of Noise in Internal Combustion Engines
Am Quoc Do
Ho Chi Minh City University of Technology and Education, Vietnam
Corresponding author. Email: amdq@hcmute.edu.vn
ARTICLE INFO
ABSTRACT
03/06/2024
Internal combustion engines are extensively used in various modes of
transportation, including ships and cars. However, their operation
generated significant noise, which can disrupt daily life and poses health
risks. Prolonged exposure to noise can lead to insomnia, cardiovascular
diseases, and even death, making noise pollution a growing social concern.
As the primary power source and major noise contributor in ships and
automobiles, internal combustion engines play a crucial role in overall
noise levels. The noise produced by internal combustion engines stems
from various sources and mechanisms within the engine. These includes
the combustion process itself, mechanical interactions between engine
components, and the exhaust system, ect. Each of these sources have
specific causes and contributing factors. This work discusses and analyzes
the sources of noise, their causes, and the factors that influence them. By
addressing these key areas, it is possible to reduce the negative effects of
engine noise on human health and well-being.
21/06/2024
02/07/2024
28/02/2025
KEYWORDS
Engine noise;
Combustion noise;
Mechanical noise;
Knock;
Engine knock.
Doi: https://doi.org/10.54644/jte.2025.1608
Copyright © JTE. This is an open access article distributed under the terms and conditions of the Creative Commons Attribution-NonCommercial 4.0
International License which permits unrestricted use, distribution, and reproduction in any medium for non-commercial purpose, provided the original work is
properly cited.
1. Introduction
Noise is unwanted or harmful sound, and is considered noisy or unpleasant to the hearing. Noise
pollution is a growing issue, exacerbated by factors like population growth, urbanization, and higher
levels of transportation. It is closely associated with health problems like elevated blood pressure,
heightened stress, tinnitus, hearing impairment, sleep disturbances, and accelerated cognitive
deterioration. Some specific kinds of loud sounds, like those generated by heavy manufacturing
(exceeding 105 dB for an hour) or firearm noise (over 130 dB for a few seconds), can result in lasting
damage to one's hearing [1]-[2].
For a healthy young individual, the audible range spans from 20 Hz to 20 kHz. The sound pressure
level (SPL) at the border of this range fluctuates with frequency. Specifically, at 1 kHz, the SPL can
vary from 0 to 130 dB. The human ear is greatest feeling within the frequency range of 500 Hz to 5 kHz,
while it tends to be less responsive to sounds below 100 Hz (see Fig 1).
Figure 1. The audible range [3], [4].
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Engine noise levels usually range from 80 to 110 dBA when measured at a distance of 1 meter from
the engine surface [5],The intensity of this noise is contingent upon factors such as engine size, speed,
and injection system. Various elements within ICE contribute to its overall noise, including exhaust
noise, intake noise, fan noise, combustion noise, piston slap noise, and valve system noise. The relative
contributions of these sources to the overall sound pressure level of engine noise (at a distance of 1 meter
from the engine) can vary significantly depending on operating conditions and the specific type of
engine, as illustrated in Fig. 2 [6].
Sources
Figure 2. Various sources contribute to the total sound pressure level of noise emitted by an engine,
(measured at a distance of one meter from the engine) [6]. (1) exhaust noise; (2) intake noise; (3) fan
noise; (4) combustion noise; (5) piston slap noise; (6) noise of accessories and belt; (7) valve system noise.
In internal combustion engines (ICE), the generation mechanism of noise can be described as
follows: When the combustible mixture undergoes compression and combustion in the engine’s
combustion chamber, it results in intense pressure changes. These dynamic loads impact all connected
components, leading to complex structural vibrations. These vibrations are then transmitted through
various engine parts, such as the cylinder cover, cylinder liner, and crank-connecting rod mechanism.
Ultimately, the external surface structure of the engine radiates noise to the surrounding environment
[7]. The noise generation process and transmission path in internal combustion engines are described in
Table 1.
The noise sources within internal combustion engines can be categorized into two primary types:
those originating from the combustion process and those resulting from mechanical impacts (refer to
Table 1) [8]-[9]. Typically, the combustion process and mechanical interactions like piston slap are the
predominant factors contributing to vibration and noise in an ICE.
Table 1. The excitation on ICEs and their noise generation process [4], [10], [11].
Force
Generation
Force Applied
to Structure
Vibration
Transmission
Noise
Radiators
Cylinder
Pressure Pulses
Cylinder Head
Rocker Cover
Manifolds
Piston
Connecting-Rod
Crankshaft
ICE Block
Mechanical
Mechanical
Impacts:
• Piston Slap
• Bearings
• Valves
• Fuel Pump
Cylinder Walls
Water Panels
Side Covers
Sump
Timing Cover
Combustion
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Noise and vibration generated in the engine are mainly radiated from the outside parts of the engine.
The radiated sound intensity P can be based on the following equation [10].
P= d.v.s.𝛍r. V2
(1)
With:
d = density of air
V = velocity of sound
s = radiation area
𝛍r= radiation efficiency
V2 =temporal and spatial average of the surface vibration velocity squared
2. Combustion noise
The noise produced during combustion stems mainly from swift shifts in internal cylinder pressure.
This combustion sequence leads to dynamic loads because of rapid pressure alterations, high-frequency
gas oscillations, vibrations, and pulse waves [12], [13]. The magnitude of noise originating from
aerodynamic loads depends on maximum of the speed-duration of pressure elevation (dp/dt) and peak
pressure. Experimental observations show that the intensity of combustion noise correlates with cylinder
pressure [12].
I~ [Pmax(dp/dt)max]2
(2)
Where:
I: the sound intensity of the combustion noise; Pmax: pressure peak in the cylinder; and (dp/dt)max: the
maximum rate of pressure rise.
The pressure in a diesel engine's cylinder exceeds that of a gasoline engine, with a higher maximum
rate of pressure increase. In particular, a diesel engine encounters a pressure increase rate ranging from
0.3 to 0.6 MN/m2/crankshaft angle, roughly three times higher than that of a gasoline engine [14]. As a
result, the noise produced by a diesel engine is significantly louder than that of gasoline engines [13]-
[15].
2.1. Knock in gasoline engine
In gasoline engines, combustion noise typically constitutes a minor portion of the overall noise.
However, when combustion knock occurs, it generates a flame front with a propagation speed that is 10
to 100 times faster than that associated with normal combustion initiated by the spark plug (which is
around 20m/s) [16]. Detonation fire depends on the time it takes to spread the flame film to a location
capable of causing detonation and the time needed to form a self-igniting center. Detonation will not
occur if the flame spreads before the unburned gas mixture has enough time to cause spontaneous
combustion [17]. This uncontrolled burning creates pressure waves that spread in circular patterns from
the heart of the reaction. Knocking occurs near the top center during combustion, leading to increased
peak pressure and pressure fluctuations in the cylinder. It is the collision of these pressure pulses with
the cylinder walls that generates the metallic pinging sound. Engine knocking can occur at various
speeds but is often not heard at high RPMs due to other engine noises.
Detonation in gasoline engines is often affected by the following key factors [16]:
- Ignition timing: Advancing the ignition timing increases combustion chamber temperature and
peak pressure.
- Cylinder-charge density: Higher torque demand requires increased charge density, leading to higher
compression temperatures.
- Fuel quality: Using low-octane fuels can raise the risk of knock so following the manufacturer's
recommended fuel grade is crucial.
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- Excessive compression ratio: Issues like a thinner cylinder head gasket can raise pressures and
temperatures during compression. The presence of deposits within the combustion chamber can
also influence the compression ratio.
- Cooling system efficiency: Poor heat dissipation can result in high temperatures in the combustion
chamber.
- Engine geometry: Unfavorable combustion chamber geometry and improper intake manifold
configuration can lead to poor mixture turbulence and swirl, increasing the engine's detonation
tendency.
2.2. Knock in diesel engine
Diesel engines operate without an externally supplied ignition spark. Instead, high-pressure fuel is
injected into the combustion chamber towards the end of the compression stroke, forming a
heterogeneous mixture. Combustion ensues when the pressure and temperature within the combustion
chamber surpass the self-ignition conditions of the fuel [17].
The period between fuel injection and combustion is termed the combustion delay time. A prolonged
delay time leads to increased fuel accumulation within the combustion chamber and a buildup of heat.
Consequently, combustion occurs rapidly, causing a sharp rise in pressure within the combustion
chamber, impacting the surrounding components. This rapid and intense combustion process manifests
as engine knocking sounds [18].
The large combustion delay time depends on the following main factors:
- Cold engine or large heat loss.
- Fuel injection timing is too early.
- Low self-ignition index of the fuel (low cetane index).
- The fuel may not be adequately atomized, or the fuel injection pressure might be insufficient.
Pre-injection effectively relieves the sudden increase in combustion pressure. When a small amount
of fuel (approximately 1 mg) is burned during the compression phase, it raises the pressure and
temperature in the cylinder. As a result, the ignition delay for the main injection is shortened, positively
impacting combustion noise [19].
2.3. The impact of various parameters on combustion noise
2.3.1. Fuel quality
In diesel engines, the cetane number significantly influences cylinder pressure development and
ignition delay. Fuels with low cetane values result in a prolonged combustion delay, leading to higher
rates of pressure rise and maximum peak pressure, ultimately increasing combustion noise.
The octane rating of a gasoline fuel indicates its resistance to detonation. A higher octane rating
corresponds to a higher resistance to detonation in the engine. During engine operation, it is necessary
to increase the octane of the fuel. This can be explained as follows, after a period of operation the soot
layer formed in the engine combustion chamber does little to change the compression ratio, but
significantly increases the temperature of the wall within the combustion chamber, thus increasing the
heat transferred to the mixture during the intake cycle. Therefore, it increases the possibility of
detonation. For car engines, after running about 15.000 - 25.000 km, the octane number needs to increase
about 5 units [17], [20].
2.3.2. Load
Essentially, as the load increases, more fuel is burned, resulting in greater heat release. This elevation
in combustion pressure peak and pressure rise rate also occurs. However, with the rise in combustion
chamber temperature, the gap between the cylinder wall and piston diminishes, potentially resulting in
reduced noise levels [12].Considering the possibility of detonation in a diesel engine, when increasing
the load on a diesel engine (using undivided combustion chamber), the combustion delay time decreases
almost linearly. It can be explained as follows: when the load increases, the combustion chamber wall
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temperature and residual gas temperature will increase, increasing the temperature of the new charge
mixture. The above effects will reduce the combustion delay time [17], [20].
2.3.3. Engine speed
In a diesel engine, if the combustion delay time is measured in milliseconds, increasing the engine
speed while maintaining the load will shorten the combustion delay time. This phenomenon can be
explained as follows: at higher engine speeds, the air leakage decreases, and fuel injection pressure rises.
Additionally, the peak temperature during compression increases due to reduced heat loss.
Consequently, the combustion delay time is shortened [17], [20].
In a spark ignition engine, at high speed, the air-fuel mixture has less time to overheat. Moreover,
the pressure at the conclusion of the compression stroke, typically decreasing with higher engine speeds,
presents fewer favorable conditions for detonation to take place. Conversely, the shorter time for
combustion heat to dissipate will increase cylinder temperature. However, under the combined influence
of the above factors, the occurrence of engine detonation tends to decrease as the engine speed increases
[21].
2.3.4. Injection parameters
There are some injection parameters expressed as pre-injection, main injection, fuel quantity,
pressure, and injection start [16]. An electronic controller unit calculates variables like temperature
coefficient, engine speed, load, and altitude to achieve this precision. In common-rail injection systems,
injection pressure remains virtually constant during the injection process. This pressure independence
from engine speed allows for better control. The double injection technique (pre-injection and main
injection) balances pressure rise and increases engine power [19].
GDI engines behave similarly to manifold-injected engines when operating with homogeneous
air/fuel mixtures. In stratified-charge mode, only the area near the spark plug tip contains an ignitable
mixture. The rest of the combustion chamber is filled with air or inert gases, eliminating the risk of
spontaneous ignition and engine knock [16].
2.3.5. Turbocharge
Turbocharging an engine elevates the temperature and pressure of the intake air, consequently raising
the temperature of the air/fuel mixture. In diesel engines, turbocharging decreases combustion delay
time, thereby reducing combustion noise, especially under full load conditions. Intercoolers, commonly
paired with turbochargers, serve to lower air charge temperature, thus reducing NOx emissions but
potentially increasing the noise level of the combustion process [22].
2.3.6. Mixture ratio
Experimental results show that different air/fuel ratios (λ) will lead to different fire film propagation
speeds. When λ is approximately 0.85 to 0.95, the flame film propagation speed reaches its maximum
value. In this case, the temperature and pressure at the end of the flame film are very large, causing the
unburned gas mixture to be strongly compressed, leading to its pressure and temperature increasing.
This will lead to detonation [17], [23].
2.3.7. Ignition and injection timing
In gasoline engines, increasing the ignition advance angle leads to a loss of work during the
compression process. Additionally, it raises the temperature of the air-fuel mixture at the end of the
flame propagation zone, thereby increasing the tendency for detonation.
When increasing the injection timing on a diesel engine, fuel is injected into the low-pressure and
low-temperature air mass, leading to a longer combustion delay time. Consequently, the rate of pressure
increase (dp/dt) and the maximum pressure (Pmax) rise, resulting in the emergence of engine noise [23].